Switched mode power supplies are frequently used to provide supply voltages to one or more circuit parts. Some switched mode power supplies employ features like power factor correction (PFC). Switched mode power supplies may provide galvanic isolation for example by using a transformer. In such switched mode power supplies, energy is selectively supplied to a primary side of the transformer by operating primary side switches with a switching frequency, often with a pulse width modulation scheme. A switching frequency of such primary side switches is regulated to obtain a desired output voltage needed for a particular application.
On a secondary side, rectifiers may be used in switched mode power supplies. Some implementation use synchronous rectifiers which use switches operated synchronously with primary side switches to provide a DC (direct current) output. Therefore, an example topology of a switched mode power supply (SMPS) system may include a power factor correction circuit, a voltage converter circuit like a LLC converter and a synchronous rectification (SR) circuit. Other types of voltage converters than LLC converters, for example other resonant converters, may also be used.
Switching of primary side switches of such SMPS systems and of synchronous rectifier (SR) switches is in many cases optimized to minimize switching losses. For example, concepts like zero voltage switching (ZVS) for primary side switches or switching at zero current for SR switches may be employed.
Nevertheless, in some situations, a shorter switching cycle, for example a shorter on time of a switch, than in normal operation may occur. One example for such a situation is when the power of a system (supply voltage) is turned off, either due to a power failure or because of a user switching off the system. In such a case, a controller (for example primary side LLC controller) may have its supply voltage reduced quickly below a threshold voltage leading to a reset. This in turn may set output voltages of gate drivers driving gates of primary switches or secondary synchronous rectifier switches to values turning the switches off immediately. Therefore, a last turn-on time at such an event may be shorter than previous turn-on times of a respective switch.
Another situation may occur in case of other fault conditions occurring. Switched mode power supply systems, in particular when used in safety critical environments, may employ various fault detection features like undervoltage detection, overvoltage detection, overcurrent detection etc. In case a corresponding fault condition occurs, switching may also be terminated immediately, leading to shorter turn-on times of one or more switches. For example, a controller for synchronous rectifier switches may be supplied by an output of an LLC converter. When the LLC output is stopped, this controller is reset which may lead to a shorter turn-on time of a synchronous rectifier switch.
Such shortened turn-on times in some situations may lead to voltage oscillations across output capacitors of synchronous rectifier switches (for example implemented as MOSFETs) and consequently to voltage spikes across the synchronous rectifier switch. Such voltage spikes may adversely affect the switch and/or may require the switch to be designed to withstand such voltage spikes, which increases an area required for implementing the switch and costs.